help/config: fix rendering of definition list

Without the blank line, the minirst parser renders

  Term-1
    Line-1

    Line-2
  Term-2
    Line-1

as

  Term-1
    Line-1

  Line-2 Term-2 Line-1

because the second term is seen as a paragraph.

mercurial/help/config.txt

@@ -1141,6 +1141,7 b' The full set of options is:'
 
     To disable SSL verification temporarily, specify ``--insecure`` from
     command line.
+
 ``cache``
     Whether to support caching in hgweb. Defaults to True.
 ``contact``